1. Field of the Invention
The invention is generally related to a system for evaluating the remaining useful life and performance characteristics of parts made from NYLON 11.RTM., a polyamide plastic made from 11-aminoundecanoic acid (hereinafter variously referred to as polyamide formed from aminoundecanoic acid or NYLON.RTM. or RILSAN.RTM.). More particularly, the invention uses molecular weight measurements to assess the amount of life used or remaining for the part, the rate at which the useable life is changing, and the date at which the part should be replaced.
2. Description of the Prior Art
Life monitoring of plastic and composite parts has become very important in modern industrial processes. Many mechanical parts, including load bearing parts, are now being made of plastic materials because they are lower weight than metals, they do not corrode like metals, they can be more easily formed into desired shapes, they stand up to extreme temperature conditions better than many metals, and for other reasons. However, plastic and composite parts do breakdown and require periodic replacement. In particular, they can become brittle and have reduced tensile strength with time due to exposure to certain chemicals, pH conditions, temperature, exposure to ultraviolet light, and for other reasons. Unlike metal parts, the deterioration of plastic parts is more difficult to assess. For example, weaknesses in metal materials can be assessed by visually detecting cracks and the like; however, cracking in plastic parts is not often a good indicator of structural integrity.
A normal practice in industry is to periodically replace plastic parts before their useful life expires. This is accomplished by conducting aging studies on representative parts, and then establishing a practice of replacing all parts prior to the expiration of a predicted life time for the part. This practice often results in replacing parts well before they are required, which is an expensive proposition in terms of both the material and the time a machine is shut down to replace the part.
Current mechanical testing practices often call for determinations of the percent of extension of a test sample under certain loads, and the load at break of the test sample. Changes in these parameters are then used to assess the fatigue of a part. For example, increases in the percent of extension can indicate that the tensile strength of a part is deteriorating, and decreases in load at break can indicate embrittlement. The difficulty with using this type of mechanical testing of plastic parts in an industrial operation is that the operator can often only tell when the part is no longer useful when it is too late. That is, a part will often perform well up until the point that it breaks and falls apart.
Recently, a few advances have been made in "life monitoring", which refers to a process of assessing the age and/or remaining useful life of a plastic part.
U.S. Pat. No. 5,305,645 to Reifsnider et al. discloses a process for dynamically measuring material strength and life under cyclic loading. Fatigue damage in a specimen is assessed by monitoring signals produced using a dynamic testing machine that applies cyclical loading to a specimen. By analyzing phase differences between displacement and load signals, as well as identifying a change in amplitude of the displacement, the life remaining and residual strength of a sample can be ascertained. Despite its advantages, the Reifsnider et al. technique suffers from the problem of requiring very expensive machinery, and requires specimens to be removed and tested.
U.S. Pat. No. 5,317,252 to Kranbuehl discloses a technique for life monitoring of a plastic part or chemical fluid by monitoring electrical permittivity parameters of a dosimeter. In Kranbuehl, "dosimeters" are described which are applied to the surface of a polymer part being monitored or are simply positioned adjacent the polymer part as a "coupon-type" sensor. In this way, the dosimeter is subjected to the same environment as the part to be monitored (e.g., corrosive fluid handling, light damage, heat damage, etc.). The dosimeter includes a capacitor which undergoes reproducible changes in one or more components of a complex permittivity measurement, and these changes are correlated to a degree of deterioration of the polymeric material being monitored. The approach described in Kranbuehl provides low cost in-situ measurements to be performed; however, it requires a sensor that is selected to undergo reproducible changes in complex permittivity components and requires correlating the changes with the deterioration of a property of interest. Both of these requirements may be difficult to satisfy in certain situations.